This disclosure relates to an image forming apparatus that includes a driven unit provided in an inner portion of a housing and a cover member configured to open and close an opening formed in the housing, and in particular, relates to an image forming apparatus that includes a cover open/closed detection sensor configured to detect an open/closed state of a cover member.
A conventional electrographic image forming apparatus forms an opening for maintenance in the housing of the image forming apparatus to facilitate removal of a sheet of paper that has jammed in the fixing device provided in an inner portion, and in addition mounts a cover on the opening. In various types of image forming apparatuses, when a user removes a printed sheet of paper by opening the cover, there is a risk of erroneously executing the printing operation relating to the user. Furthermore, there is a risk that the apparatus will malfunction due to performance of a printing operation in a state in which the cover is open. As a result, a conventional image forming apparatus includes a cover open/closed detection sensor as an interlock switch that detects whether the cover is open or closed. When the cover is open, application of current to the motor that drives the heater or the heating roller of the fixing device is suspended, and operation is stopped. In this manner, the user can be protected from contact with the heat of the heater or the heating roller, and a malfunction caused by erroneous operation of the image forming apparatus can be prevented.
In the image forming apparatus above, when the printed sheet that is jammed in the fixing device is pulled by the user in a state in which the cover is open, the heating roller is rotated by reason of the frictional force with the printed sheet that is pulled. At this time, the output shaft of the motor that supplies drive force to the heating roller is rotated and a counter electromotive force is produced by the motor. The counter electromotive force produces noise or the like in the control circuit of the control unit of the image forming apparatus. As a result, there is a risk of the occurrence of an unpredictable problem in the image forming apparatus caused by the counter electromotive force. For example, noise resulting from a counter electromotive force may have an adverse effect on the control unit with the result that a closed signal indicating that the cover is closed may be input to the control unit although the cover is not closed. In this situation, the control unit erroneously determines that the cover is closed, and executes control to drive the heater or the heating roller (hereinafter referred to as “warming-up control”). Therefore, a phenomenon is known in which the control unit makes an erroneous determination due to the effect of the noise.
Even when warming-up control is performed as a result of an erroneous determination by the control unit as described above, safety characteristics in relation to the user can be enhanced by directly cutting off power supply to the circuit that supplies a drive current to the motor or the heater by use of an electromagnetic contactor. However, when warming-up control is executed by the control unit in response to input of the closed signal, for example, a control program may be executed such as an abnormality determination program to ascertain whether or not the temperature control of the heater of the fixing device is operating normally, or an abnormality determination program that determines whether or not the heating roller is rotating normally. When the control program is executed, notwithstanding that operation is normal, a result may be output that the heater or the motor, or the temperature sensor or the rotation sensor is abnormal. In this case, since the heater or the motor is an important element in an image forming apparatus, the output of a high-level error message not only causes concern in a user of the image forming apparatus, but also since an abnormal and normal operation cannot be distinguished, the problem arises that although operation is normal, components or devices that have been determined to be abnormal must be replaced.